Indoor cacti can be damaged by close proximity to heating and air conditioning vents, and outdoor cacti might turn brown from sunburn or frost damage. All cacti can turn brown from root damage, insects and graft problems.
Hot or cold air blowing from vents dries out cactus tissue and causes it to become brittle and brown. Moving the cactus away from the vent prevents browning. Extreme high or low temperatures can lead to sunburn or frost damage and ruptures cells in the cactus. The healing process of these ruptured cells results in brown and brittle spots. Indoor cacti can also turn brown from sunburn when they come in contact with glass on a window or are placed too near a window that intensifies sunlight.
To determine if a cactus is turning brown from root damage, inspect the roots to see if they are black, brown or soft. Trim any damaged roots and prune them to undo any binding. Place the cactus in better-draining soil and reduce watering. To determine which type of insects are causing the cactus to brown, trap them or use an insecticidal soap or horticultural oils on the cactus to eliminate them. A failed graft is uncommon, but causes the cactus to brown when it forms a protective shell between the rejected areas of the plant. There is no remedy for this occurrence.